Radioactivity and the Nucleus: Physics 0625 (Cambridge O Level / IGCSE)

A student is investigating three sealed radioactive sources, labelled P, Q and R, in a school physics laboratory. A Geiger–Müller tube and counter is placed the same fixed distance from each source in turn, and different absorbers are placed between the source and the tube. The student's results are:

• Source P: detected through a sheet of paper and through 5 mm of aluminium, but not through 20 mm of lead.
• Source Q: not detected even through a single sheet of paper.
• Source R: detected through a sheet of paper, but not through 5 mm of aluminium.

Based on these results, which type of radiation is source R most likely emitting?

A water company suspects that an underground pipe is leaking somewhere along its length. To find the exact location without digging up the whole pipeline, engineers add a small amount of a radioactive tracer to the water flowing through the pipe. A detector is then moved slowly along the ground above the pipe; the count rate it records rises sharply above any point where leaked water (and therefore the tracer) has reached the soil close to the surface.

The tracer used is a nuclide represented as ${}^{140}_{57}\text{X}$.

(a) State the number of protons and the number of neutrons in a nucleus of this nuclide. [2]

(b) A different sample contains the nuclide ${}^{142}_{57}\text{X}$. Explain what is meant by saying that ${}^{140}_{57}\text{X}$ and ${}^{142}_{57}\text{X}$ are isotopes of the same element. [2]

(c) Before adding the tracer to the water, the engineers test a small fixed sample of it in the lab and record how its count rate changes with time.

Use the table to determine the half-life of the tracer. [2]

A group of students is investigating a small sealed sample of a radioactive isotope in a school laboratory. One student, Amir, suggests that if the sample were heated in an oven, or dissolved in acid to form a different chemical compound, its rate of radioactive decay would change.

(a) State what is meant by describing radioactive decay as random. [1]

(b) State what is meant by describing radioactive decay as spontaneous. [1]

(c) Use your answers to (a) and (b) to explain why Amir is incorrect. [2]

(d) The sample decays by emitting alpha particles. State, in general terms, what happens to the nucleus of an atom of the original element when it emits an alpha particle. [1]

A researcher directs a narrow beam containing alpha particles, beta particles and gamma radiation from a mixed source into the region between two oppositely charged parallel plates, held vertically with the left-hand plate positive and the right-hand plate negative. Before the field is switched on, the beam travels horizontally, midway between the plates.

Which row correctly describes what happens to each type of radiation once the electric field is switched on?

A sealed source manufactured for industrial radiography contains a freshly made, highly unstable nuclide. For this question, this nuclide is represented as ${}^{224}_{88}\text{X}$ (X is used here only as a generic label, not a real chemical symbol).

${}^{224}_{88}\text{X}$ decays by emitting an alpha particle to form a new nuclide, Y. The nuclide Y is itself unstable and decays further by emitting a beta particle to form a new nuclide, W.

(a) Write a balanced nuclide equation for the decay of X to Y, showing the nucleon number and proton number of Y and including the symbol for the alpha particle emitted. [2]

(b) Write a balanced nuclide equation for the decay of Y to W, showing the nucleon number and proton number of W and including the symbol for the beta particle emitted. [2]

(c) During the decay of Y to W, a neutron inside the nucleus changes into other particles. Describe this change, and explain how it helps to make the nucleus more stable. [2]

Background radiation is the low-level ionising radiation that is detected everywhere in the environment, even when no deliberately placed radioactive source is nearby.

Which of the following is NOT a source that contributes to background radiation?

A technician is testing a small radioactive source in a laboratory. Before placing the source near a Geiger–Müller (GM) tube, she first records the steady count rate with no source present. This background count rate is 20 counts per minute, and it stays constant throughout the test.

She then places the source close to the GM tube and records the total count rate at four different times:

(a) Explain why the total count rate recorded with the source present must be corrected for background radiation before it is used to find the half-life of the source. [1]

(b) Calculate the corrected count rate (the count rate due to the source alone) at each of the four times in the table. [2]

(c) Use your corrected values to determine the half-life of the source. [2]

A student has learned that alpha particles are stopped by paper, beta particles are stopped by a few millimetres of aluminium, and gamma rays need several centimetres of lead to be absorbed significantly. She makes the following claim:

"Because gamma radiation passes through paper, aluminium and even several centimetres of lead more easily than alpha or beta radiation, it must interact least with the materials it passes through, and for that same reason, gamma radiation must also be the safest of the three to be near."

(a) State the order of alpha, beta and gamma radiation from the most ionising to the least ionising. [1]

(b) Explain the relationship between the ionising power of a type of radiation and its penetrating power. [2]

(c) Discuss whether the student's claim that gamma radiation is "the safest of the three to be near" is fully justified. In your answer, refer to both the ionising power and the penetrating power of alpha, beta and gamma radiation. [2]

A smoke detector contains a small sealed source of the isotope americium-241, represented in nuclide notation as ${}^{241}_{95}\text{Am}$. This isotope decays by emitting an alpha particle to form a new nuclide, G. The nucleus of G is produced in an excited (higher-energy) state, and almost immediately loses its extra energy by emitting a gamma ray to reach its stable ground state.

Inside the detector, alpha particles travel across a small air gap between two electrodes, ionising the air and allowing a tiny, continuous electric current to flow. If smoke enters the gap, some alpha particles are absorbed by the smoke particles, the current falls, and the alarm is triggered.

(a) Write a balanced nuclide equation for the alpha decay of ${}^{241}_{95}\text{Am}$ to G, including the nucleon number and proton number of G and the symbol for the alpha particle emitted. [2]

(b) State the nucleon number and proton number of G after it has also emitted the gamma ray, and explain your reasoning. [2]

(c) Suggest why alpha particles, rather than beta particles or gamma rays, are suitable for use inside this small, sealed detector unit. [2]

A radioactive isotope used in a hospital scan has a half-life of 6 hours. Immediately after preparation, a sample has a background-corrected activity of 960 Bq.

Assuming no more of the isotope is added, how long after preparation will the activity first fall to 60 Bq?